REVERSE CURRENT LIMIT PROTECTION FOR ACTIVE CLAMP CONVERTERS
DC-to-DC converters are protected from damage by, among other things, monitoring and controlling forward and reverse currents in their transformer primary windings. The currents are discontinued if their values fall outside a predetermined range or if they flow during a portion of the switching cycle in a manner that would result in cross-conduction. Power switches in these converters are also protected from damage by adjusting the maximum duty cycles of these converters to vary with their input voltages. In this way, the maximum voltage across the power switches is kept within a relatively narrow range. These protective features can be combined in any number of ways to fit the application at hand.
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This application claims priority under 35 U.S.C. §119 (e) of the co-pending U.S. provisional patent application Ser. No. 61/359,752, filed Jun. 29, 2010, and titled “Reverse Current Limit Protection for Active Clamp Converter,” which is hereby incorporated by reference.
FIELD OF THE INVENTIONThe present invention relates to power electronics. More specifically, the present invention relates to DC-to-DC converters.
BACKGROUND OF THE INVENTIONForward converters with active-clamp reset offer multiple benefits to designers and are presently finding wide use. Power converters based on the forward topology are particularly useful in applications where high efficiency and good power handling capability are required, mostly in the 50W to 500W output power range. While the popularity of the forward topology is based upon many factors, designers have primarily been drawn to its simplicity, performance, and efficiency. The forward converter is derived from the buck topology, however the transformer employed in the forward topology provides input-output ground isolation as well as a step-down or step-up function. The transformer in a forward topology does not inherently reset each switching cycle as do symmetrical topologies. A number of different reset mechanisms have been employed in forward power converters, each method having its own benefits and challenges.
The voltage the clamp capacitor CC 150 measured at the bottom of the primary winding 102 drops below 0 during the OFF time of the duty cycle, calculated as (1−D)TS, as a result of the voltage stored in the clamp capacitor CC 150. The voltage is positive at the node A of
As can be seen, VDS is a function of either VIN and D, or VIN and K. D should not approach one, or VDS will increase to a point where Q1 110 may be destroyed. Therefore, the voltage across CC 150 during time t2 is also impressed across the drain-source of Q1 110. As a result, an appropriate transistor must be selected to handle this voltage. However, as is known, a transistor having a larger VDS rating, meaning it can handle higher potentials across its drain-source, is physically bigger, more expensive, and less efficient. It is desirable to keep that voltage at the time t2 as low as possible so as to be able to make Q1 110 as small as possible, thereby making Q1 less expensive, more efficient and, in terms of form factor, more desirable.
As shown in
In accordance with principles of the invention, DC-to-DC converters are protected from internal damage by monitoring and controlling currents that flow through them. Damage can result from excessive currents, from shorts generated by cross conduction, and from excessive voltages across power switches and other components. Both positive and negative currents flowing within the converters are monitored so that appropriate action can be taken before any catastrophic damage occurs. In this way, DC-to-DC converters employing principles of the invention are less susceptible to these and other damage.
In a first aspect of the invention, a DC-to-DC converter includes a transformer, a sensing element, and a controller that controls currents during switching cycles of the DC-to-DC converter. The transformer has a primary winding coupled to a forward current path and a reverse current path. The forward current path sources a forward current, from a top of the primary winding to a bottom of the primary winding during a power phase of a switching cycle. The reverse current path sources a reverse current, from the bottom of the primary winding to the top of the primary winding during a reset phase of the switching cycle. The sensing element senses the forward and reverse currents, and the controller controls these currents based on their sensed values.
In one embodiment, the controller discontinues the forward current during a remainder of the power phase if the forward current is above a predetermined first threshold and discontinues the reverse current during a remainder of the reset phase if the reverse current is below a predetermined second threshold. The DC-to-DC converter includes a power switch on the forward current path and a reset switch on the reverse current path, such that the controller automatically turns OFF the power switch when the forward current is above the predetermined first threshold and turns OFF the reset switch when the reverse current is below the predetermined second threshold.
In one embodiment, the controller controls the power switch and the reset switch to automatically adjust a duty cycle of the DC-to-DC converter to maintain a voltage across the power switch within a predetermined range. The controller uses an input voltage to the DC-to-DC converter to determine a maximum duty cycle.
In another embodiment, the controller turns ON the power switch only if a magnetizing current through the primary winding is a reverse current, and turns ON the reset switch only if the magnetizing current is a forward current.
The sensing element includes a sense resistor coupling the forward and reverse current paths to a common potential, such as ground. The controller monitors voltages or currents across the sense resistor to determine the value and direction of current flows. Alternatively, the sensing element includes first and second sense resistors coupling the forward and reverse current paths, respectively, to the common potential.
In a second aspect of the invention, a method of controlling current flow in a DC-to-DC converter that has a transformer with a primary winding includes sensing positive currents, in a forward current path, and sensing negative currents, in a reverse current path. The method includes automatically adjusting the positive and negative currents during a switching cycle of the DC-to-DC converter. As one example, the positive and negative currents are adjusted by adjusting duty cycles of the DC-to-DC converter using pulse width modulation. Preferably, the forward current path contains a power switch, and a voltage across the power switch is inversely proportional to D*(1−D), where D is a duty cycle of the DC-to-DC converter.
In a third aspect of the invention, a controller for monitoring and adjusting currents during a switching cycle of a DC-to-DC converter includes a processor that executes a state machine having at least first, second, and third states. The first state corresponds to a first current having a positive polarity (corresponding to current flowing in a positive direction) and a second current having a substantially zero value during a first phase of the switching cycle. The second state corresponds to the first current having a substantially zero value and the second current having a negative polarity during a second phase of the switching cycle. The third state corresponds to the first and second currents both having substantially zero values during remainders of the switching cycle. The relative durations of the first and second states during the switching cycle define a variable duty cycle. Transitions between the first and second states are based on values and polarities of the first and second currents, on values of an input voltage to the DC-to-DC converter, or on some combination of these elements.
In accordance with principles of the invention, DC-to-DC power converters are protected from damage by, among other things, preventing currents in their transformer windings from reaching unacceptable levels, by preventing voltages across their power switches from exceeding predetermined limits, and by preventing short circuits. These DC-to-DC power converters prevent these failures by monitoring and adjusting both forward and reverse currents. These converters use smaller, more reliable, and less expensive power switches, are more robust, and are able to be used in a wider range of applications.
The converter 300 has two current-sensing points CS 325 and CS2 335, each having a current sensing resistor. The current flowing from the transformer 305 through the sense resistor RS 315 is measured at the point CS 325. The measurement can, for example, be of the voltage across RS 315. The reverse current flowing from ground up through Q2 330 is measured at the point CS2 335 as the voltage across RS2 320.
A controller 360 receives the sensed current data from nodes CS 325 and CS2 335 and generates the driving signals NDRV and GAUX to control Q1 310 and Q2 330, such as by turning one ON and the other OFF or by turning both OFF. The controller 360 is coupled to an oscillator or some other means (not shown) for generating a clock signal and thus the switching cycle. The driving signals NDRV and GAUX control Q1 310 and Q2 330, respectively, according to both the forward current measured at the node CS 325 and the reverse current measured at the node CS2 335. As mentioned above, as the reverse current increases, thereby causing the voltage at node CS2 335 to go more negative, Q2 330 can be turned OFF independently, thereby forming a tri-state condition at Node A.
It will be appreciated that currents can be measured using voltage measurements and resistor values or by using current measurements directly. It will also be appreciated that when the bottom end of the resistors RS 315 and RS2 320 are grounded, some of the measured voltages and currents are negative.
While in the state 520, if during the interval (1−D)TS the voltage at the node CS2 (VCS2) does not fall below a predetermined negative threshold (VTH2), state 510 is entered. If during the interval (1−D)TS VCS2 falls below VTH2, the state 540 is entered. In the state 540, Q2 330 is turned OFF and, after the remainder of the interval (1−D)TS has elapsed, the state 510 is entered.
As one example, VTH1 is slightly less than the value corresponding to the saturation current of the transformer, and VTH2 equals −VTH1. In general, VTHX is set depending on scale factors such as RS (e.g.,
As for all the state diagrams relating to the controller 360, the controller 460 in
In accordance with other embodiments, cross conduction between the main power switch Q1 and the auxiliary switch Q2 is reduced or eliminated.
In the state 630, after a time (1−D)TS has elapsed, the state 640 is entered, in which Q2 330 is turned OFF. While in the state 640, if the magnetizing current ILM is negative, the state 610 is entered, but if ILM is positive, the state 650 is entered. In the state 650, after a time DTS has elapsed, the state 630 is entered.
In accordance with one embodiment of the invention, a maximum duty cycle of a DC-to-DC power converter is clamped based on the voltage VIN input to the DC-to-DC power converter. The graph 700 in
Referring again to
To prevent these problems, embodiments of the invention clamp the feed forward maximum duty cycle, meaning that VIN is taken into consideration when adjusting the duty cycle. As described below, this results in a VDSMAX that is no longer a direct function of VIN.
The graph 810A represents VDS vs. VIN for the prior art method. Since D=0.61, clamping DMX at 10% headroom gives a DMX of (0.61)(1.1) or DMX=0.67. The maximum VDS is therefore 57/(1−0.67)=172V. For VIN in a range 30V-57V, the main power switch should be rated well above 170V. Such a power switch will be large, costly, and highly inefficient.
The graph 810B shows VIN vs. VDS in accordance with one embodiment of the invention. Using the feed forward methodology, VDSMAX=VIN/(1−(KY/VIN), where K=(NP/N5) VOUT. Assuming a 10% headroom and (NP/NS)=(11/3), KY=(20.17), VDSMAX=57/(1−(20.17/57))=89V, occurs at VIN=57V. VDSMAX=301 (1−(20.17/30))=91.5V, occurs at VIN=30V. VDS stays relatively constant throughout the entire input range for 30V-57V, ranging from 91.5V to 80.7V. As a result, the maximum VDS rating on the main power switch can be nominally above 100V, accounting for manufacturing and safety tolerances. What results is a more efficient and cost effective DC-to-DC converter overall, having reduced cross conduction and a manageable reverse current and reduced danger of flux saturation within the transformer.
The controllers 360 and 460 can be application specific integrated circuits (ASICs); computer-readable media containing computer-executable instructions for executing algorithms or state machines, such as those shown in
While different embodiments have been discussed separately, it will be appreciated that the features of the different embodiments can be combined in different ways. For example, one embodiment incorporates the features of reverse-current limit protection and maximum duty-cycle clamping as illustrated in
Active-clamp controllers in accordance with embodiments of the invention are described in the document titled “Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers,” 19-5331; Rev 1; 9/10, for the products “MAX5974A/MAX5974B/MAX5974C/MAX/5974D,” and available from Maxim Integrated Products, 120 Gabriel Drive, Sunnyvale, Calif. 94086, U.S.A. The document titled “Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers” is hereby incorporated by reference.
In operation, forward and reverse currents on a DC-to-DC converter are monitored. If either of the currents falls outside a pre-determined range, that current is automatically interrupted by changing a duty cycle of the DC-to-DC converter. If one of the currents flows in the “wrong” direction during a particular part of a switching cycle, that current is interrupted to prevent cross-conduction. The maximum duty cycle is also dynamically calculated to ensure that a voltage across a power switch is kept within acceptable limits.
While the invention has been described with reference to numerous specific details, one of ordinary skill in the art will recognize that the invention can be embodied in other specific forms without departing from the spirit of the invention. Thus, one of ordinary skill in the art will understand that the invention is not to be limited by the foregoing illustrative details. It will be readily apparent to one skilled in the art that other modifications may be made to the embodiments without departing from the spirit and scope of the invention as defined by the appended claims.
Claims
1. A DC-to-DC converter comprising:
- a transformer having a primary winding coupled to a forward current path and a reverse current path, the forward current path configured to source a forward current, from a top of the primary winding to a bottom of the primary winding during a power phase of a switching cycle of the DC-to-DC converter, and the reverse current path configured to source a reverse current, from the bottom of the primary winding to the top of the primary winding during a reset phase of the switching cycle;
- a sensing element configured to sense the forward and reverse currents; and
- a controller configured to control the forward and reverse currents based on sensed values thereof.
2. The DC-to-DC converter of claim 1, wherein the controller is further configured to discontinue the forward current during a remainder of the power phase if the forward current is above a predetermined first threshold and to discontinue the reverse current during a remainder of the reset phase if the reverse current is below a predetermined second threshold.
3. The DC-to-DC converter of claim 2, further comprising a power switch on the forward current path for controlling the forward current and a reset switch on the reverse current path for controlling the reverse current, wherein the forward current path is parallel to the reverse current path.
4. The DC-to-DC converter of claim 3, wherein the controller is further configured to automatically turn OFF the power switch when the forward current is above the predetermined first threshold and to turn OFF the reset switch when the reverse current is below the predetermined second threshold.
5. The DC-to-DC converter of claim 3, wherein the controller is operable to control the power switch and the reset switch to automatically adjust a duty cycle of the DC-to-DC converter to maintain a voltage across the power switch within a predetermined range.
6. The DC-to-DC converter of claim 5, wherein a maximum of the duty cycle is based on an input voltage to the DC-to-DC converter.
7. The DC-to-DC converter of claim 3, wherein the controller is further configured to turn ON the power switch only if a magnetizing current through the primary winding is a reverse current, and to turn ON the reset switch only if the magnetizing current is a forward current.
8. The DC-to-DC converter of claim 1, further comprising a capacitor on the reverse current path, wherein the controller is further configured to charge the capacitor from the primary winding during the power phase and to discharge the capacitor to the primary winding during the reset phase, the capacitor having a value larger than an input voltage of the DC-to-DC converter.
9. The DC-to-DC converter of claim 1, wherein the sensing element comprises a sense resistor coupling the forward and reverse current paths to a common potential, the controller further configured to monitor voltages or currents across the sense resistor.
10. The DC-to-DC converter of claim 9, wherein the common potential is a ground.
11. The DC-to-DC converter of claim 1, wherein the sensing element comprises first and second sense resistors coupling the forward and reverse current paths, respectively, to a common potential, the controller further configured to monitor voltages or currents across the first and second sense resistors.
12. The DC-to-DC converter of claim 10, wherein the common potential is a ground.
13. A method of controlling current flow in a DC-to-DC converter comprising a transformer with a primary winding, the method comprising:
- sensing positive currents, in a forward current path from a top of the primary winding to a bottom of the primary winding, and negative currents, in a reverse current path from the bottom of the primary winding to the top of the primary winding, wherein the forward current path contains a power switch; and
- automatically adjusting the positive and negative currents during a switching cycle of the DC-to-DC converter.
14. The method of claim 13, wherein automatically adjusting the positive and negative currents comprises interrupting a positive current during a remainder of a power phase of the switching cycle if the positive current is above a predetermined positive threshold and interrupting a negative current during a remainder of a reset phase of the switching cycle if the negative current is below a predetermined negative threshold.
15. The method of claim 13, wherein adjusting the positive and negative currents maintains a maximum voltage across the power switch within a predetermined range.
16. The method of claim 15, wherein the predetermined range is below a maximum rating of the power switch.
17. The method of claim 15, wherein adjusting the positive and negative currents comprises adjusting duty cycles of the DC-to-DC converter.
18. The method of claim 17, wherein adjusting the duty cycles uses pulse width modulation.
19. The method of claim 13, wherein a voltage across the power switch is inversely proportional to a product of a duty cycle of the DC-to-DC converter (D) and (1−D).
20. The method of claim 13, the method further comprising conducting positive currents only if a magnetizing current on the primary winding is a negative current and conducting negative currents only if the magnetizing current is a positive current.
21. The method of claim 13, further comprising:
- storing a charge on the reverse current path during the power phase; and
- discharging the charge along the reverse current path during the reset phase to thereby reduce a magnetizing inductance on the primary winding.
22. The method of claim 13, further comprising detecting a voltage across one or more sense resistors to determine a direction of a magnetizing current across the primary winding.
23. The method of claim 13, wherein the reverse current path comprises a reset switch, the method further comprising detecting a voltage across one or both of the power switch and the reset switch to determine a direction of a magnetizing current across the primary winding.
24. A controller for monitoring and adjusting currents during a switching cycle of a DC-to-DC converter, the controller comprising:
- a processor configured to execute a state machine having at least first, second, and third states, the first state corresponding to a first current having a positive polarity and a second current having a substantially zero value during a first phase of the switching cycle, the second state corresponding to the first current having a substantially zero value and the second current having a negative polarity during a second phase of the switching cycle, and the third state corresponding to the first and second currents having a substantially zero value during remainders of the switching cycle, wherein transitions between the first and second states are based on values and polarities of the first and second currents, the relative durations of the first and second states during the switching cycle defining a variable duty cycle.
25. The controller of claim 24, wherein the processor is configured to receive input signals corresponding to the first and second currents and to generate output signals for controlling the first and second currents according to the state machine.
26. The controller of claim 24, wherein the variable duty cycle is based on a third value corresponding to an input voltage to the DC-to-DC converter.
27. The controller of claim 26, wherein the state machine is configured to automatically adjust the duty cycle (D) to maintain a value (1/(D*(1−D)) within a predetermined range.
28. The controller of claim 27, wherein the predetermined range is based on a predetermined drain-to-source voltage of a power switch for the DC-to-DC converter.
29. The controller of claim 24, wherein a first transition from the first state to the third state is triggered when the first current exceeds a predetermined positive threshold and a second transition from the second state to the third state is triggered when the second current is below a predetermined negative threshold.
30. The controller of claim 24, wherein a transition from the first state to the third state is triggered when the second current has a negative polarity and a transition from the second state to the third state is triggered when the first current has a positive polarity.
Type: Application
Filed: Jun 20, 2011
Publication Date: Dec 29, 2011
Patent Grant number: 8593830
Applicant:
Inventor: Thong Anthony Huynh (Fremont, CA)
Application Number: 13/164,689
International Classification: H02M 3/335 (20060101);